I think you meant to say Alt+Enter instead of Control+Enter, since Control+Enter is used to send text without processing it as a command if it is prefixed with /. If I recall correctly, at one time mIRC did use Shift+Enter instead of Alt+Enter, however users reported that it was too easy to press Shift+Enter while entering text, since Shift is also used to capitalize text. So it was subsequently changed to Alt+Enter.
If you type more text than the window can handle, you can view it by scrolling up or down with the cursor keys. This is how the multi-line editbox in previous versions worked as well in the past. I'll take a look at adding a vertical scrollbar in a future version though.
The /editbox command has always chopped off text at the first CR or LF. The reason for this is that CR LF and CRLF are all treated in a special way by editboxes, and differently depending on whether you are using a normal or richedit editbox, and whether it is a single/multi/auto editbox. So it's a lot more complicated than just pasting a $crlf $cr or $lf into it.
